National Geographic : 1967 Jan
business had lost a capable chef. Paul laid the proof on my plate, then point ed a fork at the fragrant smoke curling upward from the grill. "There is a miniature thermal," he said with a twinkle behind his glasses. "And that flake of ash drifting up in it is a sailplane. It's heavier than air, so gravity pulls it down. But the smoke is rising faster than the sinking speed of the ash, so the flake goes up." I watched our tiny "sailplane" climb, then drift off to a landing as Paul covered its thermal source with an other steak. "Any fixed-wing aircraft must move forward to keep flying," he continued. "Powered planes use engines. Sail planes use gravity; they continuously coast downhill through the air. But the sun's heat furnishes altitude-gaining lift-'green air' to a soaring pilot. Solar energy warms the earth, and the ground heats the air above it. That leads to convection-the lifting of the air in rising warm currents." As any airline passenger knows, this world is a multicolored patchwork quilt when viewed from aloft. A dark patch-a plowed field, for example grows hot in the afternoon sun. The air over it begins to rise in a huge, in visible column. The thermal cools slowly as it rises, but so does the surrounding atmos phere. Often the atmosphere cools faster with altitude than the thermal does, so the rising air becomes pro gressively warmer in relation to the air around it. As a result, the thermal's upward speed increases; it may exceed 1,000 feet a minute. The warm air may rise 20,000 feet before dissipating. Unless the air is quite dry, a point (Continued on page 58) Swooping over the Chemung Valley near Elmira, New York, Carris and Kristof per form exuberant acrobatics for the remotely controlled camera. Sailplanes have shared Elmira's skies since German glider pilot Wolf Hirth made the first U. S. cross country flight from there in 1930. In the valley below, the Schweizer Aircraft Cor poration builds more sailplanes than all other American manufacturers combined.